The usage fee for using the image forming apparatus such as copier or printer is usually calculated based on a number of pages of document that are copied or printed and a unit charge fee that is previously set for copying or printing one page of document. More recently, as described in Japanese Patent Application Publication No. 2006-301016A, the usage fee for using the image forming apparatus is calculated based on a specific type of service provided by the image forming apparatus according to a user instruction. For example, Japanese Patent Application Publication No. 2006-301016A sets a unit charge fee respectively for full-color printing and monochrome printing such that the user is charged with higher fee for full-color printing.
In addition to full-color printing and monochrome printing, the recent image forming apparatus is provided with a function of printing a document using a limited number of colors, specifically, using more than one color but less than a number of colors used for full-color printing. In case of printing the document using the limited number of colors, the image forming apparatus described in Japanese Patent Application Publication No. 2006-301016A counts a number of pages of document copied or printed using a limited number of colors, either as a number of pages of document copied or printed in full-color or as a number of pages of document copied or printed in monochrome. Accordingly, the user may be charged with higher fee when the unit charge fee for full-color printing is used, while the user may be charged with lower fee when the unit charge fee for monochrome printing is used.